1. Field of the Invention
The present invention relates to high frequency power distributors/synthesizers and, especially, to improvements in the isolation characteristic of h-f power distributors/synthesizers.
2. Description of the Related Art
Recently, an increasing number of h-f power distributors/synthesizers are being used to divide or combine h-f power in digital mobile phone stations. There is a demand for good isolation characteristic for such h-f power distributors/synthesizers.
Japanese patent application Kokai No. 123201/86 discloses such a h-f power distributor/synthesizer as shown in FIG. 2. This h-f power distributor works as both distributor and synthesizer, but description is made on only the aspect of a distributor. The distributor includes an input terminal 11, two distribution terminals 12 and 13, and a main circuit between them. The main circuit has a configuration of the so-called Willkinson distribution circuit which consists of two 1/4 wavelength long lines 22 and 23 which branch out of a line 21 connected to the input terminal 11 and a balance resistor 24. The distributor further includes an auxiliary circuit 25 connected between the distribution terminals 12 and 13. The auxiliary circuit 25 consists of a serial connection of a variable capacitor, a variable phase shifter, and a variable coupling capacitor.
When h-f power is fed to the distribution terminal 12 where a load of 50 ohms is connected to the input terminal 11, a leak electric wave damped by 20-30 dB is output at the distribution terminal 13 if the auxiliary circuit 25 does not work as a regulator. Thus, the coupling degree of the variable capacitor of the auxiliary circuit 25 is adjusted so that an amplitude is equal to that of the leak electric wave and the variable phase shifter is adjusted so that the electric waves have opposite phases. As a result, the electric waves from the main circuit and the auxiliary circuit 25, respectively, in the same amplitude and opposite phase are synthesized at the distribution terminal 13 to offset each other, thereby providing a coupling attenuation of 60 dB or more.
Such a conventional h-f power distributor/synthesizer has a good isolation characteristic but requires a complicated auxiliary circuit including a variable capacitor and a variable phase shifter. Also, the improvement in the isolation characteristic is obtained at the expense of the optimum insertion loss. Thus, the conventional h-f power distributor/synthesizer is not suitable for use in digital mobile phone stations.
The above Japanese patent also proposes to provide an isolator at the distribution terminal to improve the isolation characteristic of the Willkinson distributor so that the input signal is not leaked to the other distribution terminal. This method, however, requires an additional component or isolator, making the system more complex. Also, this method disturbs the insertion loss so that it is difficult to improve the isolation characteristic while keeping the optimum insertion loss.
The Willkinson distributor regulates the isolation characteristic by impedance match so that it is difficult to improve the isolation characteristic with the insertion loss kept optimum.